Field of the invention
The present invention relates to a monostable multivibrator. More specifically, the present invention relates to a monostable multivibrator which is used in a video or sound reproducing circuit in a VTR, for example, and formed in a monolithic integrated circuit (IC) along with a voltage controlled oscillator (VCO).
Description of the Prior Art
In a conventional VTR, especially in an 8 mm VTR, since spike noises occur in a demodulated output at a time point when two rotated video heads are changed and at a time point when a drop-out occurs, the demodulated output is processed by a holding circuit for holding a previous value. As disclosed in Japanese Utility Model Laid-open No. 62-205 [G11B5/027], a head changing noise can be eliminated by generating a pulse having a metastable period for a predetermined time period by a monostable multivibrator in response to a head changing pulse, that is, a so-called RF-SW pulse and by compensating the demodulated output by an output of holding circuit in the metastable period. As a monostable multivibrator used for such a purpose, a very high-accuracy one is required.
A circuit diagram of one example of the above described high-accuracy monostable multivibrator is shown in FIG. 2. In FIG. 2, a monostable multivibrator 101 includes an R-S flip-flop (RS-FF) 102, a comparator 103, a transistor 104, and a charge/discharge circuit 107 composed of a resistor 105 having a resistance value of R and a capacitor 106 having a capacitance value of C.
An RF SW pulse as shown in FIG. 3A is first input to a trigger pulse generating circuit 80 from which a trigger pulse as shown in FIG. 3B is generated at a leading edge and a trailing edge of the RF-SW pulse to be supplied to a set input S of the RS-FF 102.
Since an inverted output of the RS-FF 102 is supplied to a base of the transistor 104, the transistor 104 is turned-off synchronously with the trigger pulse.
The transistor 104 has an emitter connected to the ground and a collector connected to a connection point of the resistor 105 and the capacitor 106 which are connected in series between a voltage (+Vcc) line and the ground, and therefore, if the transistor 104 is turned-on, the capacitor 106 is not charged, but the capacitor 106 is brought into a charging state if the transistor 104 is turned-off.
Therefore, an input level of a + side of the comparator 103 which is connected to the connection point of the resistor 105 and the capacitor 106 is equal to a charged voltage of the capacitor 106, and the same gradually increases synchronously with the trigger pulse as shown in FIG. 3C. When the + side input level reaches a reference level Vr being applied to a - side input of the comparator 103, as shown in FIG. 3D, an output is generated from the comparator 103 to be supplied to a reset terminal of the RS-FF 102.
A non-inverted output of the RS-FF 102 is generated so as to have a pulse width of .tau. as shown in FIG. 3E, which becomes an output of the monostable multivibrator 101. Since the + side input level of the comparator 103 is equal to a charged voltage of the capacitor 106, the pulse width .tau. which is equal to a metastable period of the monostable multivibrator is given by the following equation (200). EQU .tau.=-CR ln (1-Vr/Vcc) (200)
In the equation (200), the reference level Vr and the power source voltage Vcc are constant values, respectively. Therefore, Vr/Vcc is also a constant value, and the pulse width .tau. is dependent on the values of C and R.
In cases where a high-accuracy monostable multivibrator as described above is formed in a monolithic IC, it is impossible to form a CR circuit having a high-accuracy and good temperature characteristic. Therefore, a resistor and a capacitor for the charge/discharge circuit must be connected externally. In addition, a resistor and a capacitor normally have accuracies of .+-.5%. If higher accuracy is required, it is necessary to use a resistor and a capacitor extremely high in cost.